Classical liquid phase peptide synthesis involves the well-known problems of many manipulations and considerable loss of material during the isolation and purification of intermediates.
Some of these problems have been circumvented by using continuous liquid phase techniques. Furthermore, numerous advances made in solid phase peptide synthesis have led to progress in liquid phase synthetic methods. For example, the protecting groups developed in solid phase synthesis are also useful in liquid phase synthesis.
The two major routes in liquid phase synthesis are (1) use of the t-butyloxycarbonyl protecting group ("Boc") to block the non-side chain amino functionality, and (2) use of the 9-fluoroenyl methoxycarbonyl protecting group ("Fmoc") to block the same functionality. For examples, see Thierry, J. et al. J. Med. Chem. 33:2122 (1990) and Hoeg-Jensen, T. et al. Tetrahedron Letters 32:6387 (1991).
No continuous liquid phase peptide synthetic methods using Fmoc as the protecting group and a substituted carbodiimide as the coupling agent have hitherto been reported.